1. Field of the Invention
The invention relates in general to the method for erasing a memory, and more particularly to the method for erasing a flash EEPROM.
2. Description of the Related Art
The flash EEPROM (Electrically Erasable Programmable Read Only Memory) has the advantages of preserving data with no power supplied, and the ability of erasing and writing data. FIG. 1 is a diagram showing a memory unit of the flash EEPROM. The memory unit stores a 1-bit data. The memory unit includes a control gate CG, a floating gate FG, a source S, and a drain D. The data stored in the memory unit is determined by the amount of charges contained in the floating gate FG. For example, the memory unit stores the data of 0 if the floating gate FG contains large amount of charges, and accordingly a high threshold voltage, such as higher than 5 V, is needed to apply to the control gate CG to electrically connect the source S and the drain D. The memory unit stores the data of 1 if the floating gate FG contains small amount of charges, and accordingly a low threshold voltage, such as lower than 3.2V, is needed to apply to the control gate CG to electrically connect the source S and the drain D.
The approach to write a data of 0 to the memory unit is to apply a high voltage, such as 10V, to the control gate CG, a voltage of 6V to the drain D, and a voltage of zero to the source S. Accordingly, a large amount of charges are put into the floating gate FG, and thus the threshold voltage becomes high. The approach to write a data of 1 to the memory unit is to apply a negative voltage, such as −11V, to the control gate CG and a voltage of 3V to the source S. Accordingly, the charges in the floating gate FG are removed, and thus the threshold voltage becomes low.
FIG. 2 is an exemptary diagram of the threshold voltage distribution. The Y-axis represents voltage, and the X-axis represents number of the memory units. The threshold voltages of the memory units located at the area H are larger than 5V, and thus each memory unit stores data of 0. The threshold voltages of the memory units located at the area L are lower than 3.2V, and thus each memory unit stores data of 1. The approach for reading the data stored in a memory unit is to apply a reading voltage, such as 4V, to the memory unit. Subsequently, the memory unit located at the area H is turned on, and the memory unit located at the area L is not turned on. Then, the data stored in a memory unit can be determined according to the magnitude of the current flowing by the memory unit.
The method for erasing a flash EEPROM includes steps of pre-programming, erasing, and soft-programming. In the step of pre-programming, the data of 0 is written to memory units of the flash EEPROM for stabilizing the step of erasing. In the step of erasing, the data of 1 is written to each memory units. Nevertheless, excessive charges are possibly removed from some of the floating gates because the erasing step is performed on a block of memory units at the same time. Accordingly the threshold voltages of some memory units are too low, or even below zero. Thus, the step of soft-programming is required to tighten the distribution area L for avoiding the too-low threshold voltages. The step of soft-programming is performed by applying a voltage of 3V to the control gate CG and a voltage of 5V to the drain D.
FIG. 3 is a flowchart of a well-known method for erasing a flash EEPROM. The flash EEPROM is first pre-programmed in step 220 and then erased in step 230. The step of determining if the step of erasing is succeeded is performed in step 240. If the flash EEPROM is erased successfully, the procedure proceeds to step 250; otherwise it returns to step 230 for re-erasing. In step 250, the flash EEPROM is soft-programmed and then the method ends.
A large current may be needed while processing the soft-programming step. Because the soft-programming step is performed after the success of the erasing step is ensured. The erasing step may possibly be processed for several times until success, which may generate a large number of memory units with too-low threshold voltages. Accordingly, the soft-programming step requires a large current to tighten the distribution of the threshold voltages. However, the flash EEPROM is commonly used in the low-voltage environment, which may not supply such a large current.